Apparatus is known which requires holding and rotating an article such as a workpiece for a work step to be performed on or by the article or workpiece. Examples of such apparatus are lathes, grinders, drills and electrical discharge boring machines. In many applications, precisely centered holding of the workpiece is critical, and certain electrical discharge machines currently offer centering to within 0.0003 inches accuracy.
Certain work operations require consistent accuracy much more precise than 0.0003 inches, such as boring a supercentered hole in the tip of a workpiece. Currently, such a workpiece may have a relatively large (such as 0.5 inches) diameter concentric cylindrical surface resulting from precision casting or milling or both, but requires a more precisely centered very small diameter bore than is presently achievable. It is desirable to be able to drill this supercentered bore to an extremely accurate, very small diameter centered on an axis with respect to the larger diameter cylindrical surface.
It may also be desirable to form on the tip of such a workpiece a supercentered very small diameter needle tip, centered on an axis with respect to the larger diameter concentric cylindrical surface as a reference surface. Both the boring of a hole and the creating of a needle can be performed with an electrical discharge machine, provided tooling can be provided which enables supercentering of the workpiece with respect to the electrode of the EDM. Usually the electrode of the EDM is clamped to a rotating upper cylinder, and the workpiece is affixed to a lower X-Y reference table positioned very accurately with respect to the electrode and adjustable to a very accurate degree in the horizontal plane before and during the electrical discharge operation. Tolerances for current EDM machines typically reach an accuracy of 0.0003 inches, but a need is seen for even much better accuracy.